1. Field of the Invention
The present invention relates to a CELP (Code Excited Linear Prediction) coder and, more particularly, to a CELP coder giving consideration to the influence of an audio signal in non-speech signal periods.
2. Description of the Background Art
It has been customary with coding and decoding of speech to deal with speech periods and non-speech periods equivalently. Non-speech periods will often be referred to as noise periods hereinafter simply because noises are conspicuous, compared to speech periods. A speech decoding method is disclosed in, e.g., Gerson and Jasiuk "VECTOR SUM EXCITED LINEAR PREDICTION (VSELP) SPEECH CODING AT 8 kbps", Proc. IEEE ICASSP, 1990, pp. 461-464. This document pertains to a VSELP system which is the standard North American digital cellular speech coding system.
Japanese digital cellular speech coding systems also adopt a system similar to the VSELP system.
However, a CELP coder has the following problem because it attaches importance to a speech period coding characteristic. When a noise is coded by the speech period coding characteristic of the CBLP coder and then decoded, the resulting synthetic sound sounds unnatural and a nnoying. Specifically, codebooks used as excitation sources are optimized for speech. In addition, a spectrum estimation e rror derived from LPC (Linear Prediction Coding) analysis differs from one frame to another frame. For these reasons, the noise periods of synthetic sound coded by the CELP coder and then decoded are much removed from the original noise, deteriorating commun ication quality.